Coating



P. w. PAT'roN Er AL 2,909,148

oa. zo, 1959 comme Filed sept. 1s. 195s mil/MM Val.

NLIQ DHH/JV WLM, JMW. mrm. W50

United States Patent O COATING Paul W. Patton, Valley Falls, RJ., and Robert H. Kelly, Wellesley, and James P. MacNel, Framingham, Mass., assignors to National Research Corporation, Cambridge, Mass., a corporation of Massachusetts Application September 13, 1956, Serial No. 609,736

2 Claims. (Cl. 118-49) This invention relates to coating and more particularly to vacuum deposition processes and apparatus which are particularly usef-ul in coating a substrate with cadmium.

Prior art methods of coating with cadmium have produced coated products which often have been of poor quality and which have always involved a relatively great expense.

Accordingly, it is a principal object of the instant invention to provide improved apparatus for uniformly coating a substrate with cadmium at a reasonable cost.

Another object of the .present invention is to provide improved apparatus for uniformly coating a relatively large number or" objects both at low cost and at high speed.

The invention accordingly comprises the apparatus possessing the construction, combination of elements and arrangements of parts which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.

For a fuller understanding of the nature of objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing which is a diagrammatic, schematic illustration the preferred embodiment of the invention.

The present invention is particularly directed to improved apparatus for the vapor deposition coating of substrates with cadmium. For convenience of illustration, the invention will be described initially in connection with a preferred form of apparatus designed for the coating of selenium-coated plates which are subsequently to be formed into rectiers, with a uniform layer of vapor-deposited cadmium. This specific embodiment of they invention is given for illustrative purposes only and is not to be construed as limiting the scope of the inl vention.

As heretofore practiced, the coating of such rectifier plates with cadmium by vapor deposition techniques, even when carried out with extreme care, has left much to be desired. Oftentimes, although al1 reasonable care was exercised -dun'ng the coating operation, large numbers of cadmium-coated articles have had to be classed as rejects due either to the poor quality of the cadmium coating or damage to the intermediate layer of selenium. Additionally, complicated jigs have been employed to hold the plates in place during coating. These jigs have necesSitated expensive loading and unloading operations and have enormously complicated the problem of uniformly cooling the substrate during the coating operation.

In one preferredv embodiment of the invention, the principal problems of the prior art are overcome by providing a plurality of vacuum chambers in each of which a substrate to be coated, such as a selenium-coated alu- ICC The individual vacuum chambers are preferably mounted on a rotary support, such as a carousel, Vso that each chamber is arranged to be sequentially brought to a loading and unloading position where a single operator can supervise a large number ofcoating chambers. Accordingly, as each coating chamber approaches the operator, it is opened, preferably automatically; the opera-l tor removes the coated plate and places an uncoated plate on the carrier. The chamber is then closed, also preferably automatically, and the coating chamber is moved away vfrom the operator to start its cycle, the pumpdown of the chamber being commenced as soon as the chamber has been reclosed.

The quality of the overall coating ofV cadmium and selenium is largely dependent upon the temperature of the selenium coated substrate and the temperature of the cadmium vapor. In Vthe instant invention, the temperature across the substrate to be coated should be maintained at a maximum of 70 F. This temperaturelimi tation is notk imposed by the coating characteristics of the cadmium. Rather, the temperature restriction is necessitated by the fact that the crystalline structure of the intermediate layer of selenium on the substrate undergoes a metamorphosis, attemperatures above 70 F., to a less desirable crystalline form. The maintenance Vof a substrate temperature of less than 70 F. is Ypreferably accomplished by utilizing as a cooling unit the carrier plate upon which the substrate rests while in the coating chamber. The temperature of this plate, and therefore, the temperature of the surface of the substrate can be easilyand closely regulated and the relatively large cooling surface of the carrier plate insures a substantially uniform transfer of heat from the substrate to the cooling plate across the entire surface to beV coated. As a result the surface of the substrate to be coated is' easily maintained ata temperature of less than 70 F.

In a preferred embodiment-of the invention a support means and a pressure chamber are utilized to promote intimate contact between the cooling plate and the substrate. This arrangement exploits the pressure differential existing between the atmosphere and the coating chamber after evacuation, so las to utilize the higher pressure existing outside the chamber to push the carrier plate bearing the substrate against a support. This support in one embodiment of the invention, engages only the very edges of the substrate, but, nevertheless, insures intimate w contact between the cooling surface of the carrier plate and the substrate.

The cr-ucible supporting the cadmium, on the other hand, must be heated to a temperaturebetween 400 and 500 C. For this purpose, a plurality of resistance-heated rods areprovidedwithin the walls of the cruci-ble.

Y These rods serve as heating units and can maintain a minum plate, is placed below `a source of cadmium va- Y vpors. The substrate is supported in the chamber by means closely controlled high temperature in the cadmium and its vapors if it is necessary. Accordingly, the vapors and the subst-rate are maintained at optimum temperatures for the particular type of coating Vto be done.

The cadmium vapor source, due to its relatively high heat capacity, should be maintained at a constant ten1- perature during the entireV coating cycle, even during the period when the coating chamber is open'to the atmosphere i-f it is -desired to coat a plurality of articles at high speed. However, since cadmium readily oxidizes at high temperatures, oxide contamination of the cadmium vapor source is a major problem. Therefore, in one embodiment the problem is circumvented by allowing the cadmium vapor source to cool before the chamber is opened to the atmosphere and the coated substrate removed. Before the coating operation can again commence, however, it is, of course, necessary to reheat the cadmium vapor source subsequent tothe reloading and evacuation of the chamber. However, these periods of first cooling and then reheating are time consuming. Therefore, in a preferred embodiment of the invention, a small inner crucible is placed within the cadmium vapor source and is charged with enough cadmium to fully complete only one cycle ofthe coating operation. In this Way, the cadmium in the inner crucible within the cadmium vapor source can be evaporated to dryness bef'ore the coating chamber is opened. It then lbecomes unnecessary to cool the cadmium vapor Source since it contains no cadmium which could oxidize. When, subsequently the coating chamber is opened, the empty inner crucible is removed and a cold Crucible charged with only enough cadmium to complete one coating cycle is placed within the cadmium vapor source. The coating chamber is then reclosed and quickly evacuated; consequently, the cold inner crucible and the cadmium therein do not have time to heat up and vaporize before the coating chamber is again at a low pressure; and, therefore, no oxidation and contamination of the vapor source can occur. In thisgway, it becomes possible to maintain the cadmium vapor source temperature constant thereby reducing the time losses between the actual operating cycles to a minimum.

Referring now to the drawing, there is shown a sectional, largely schematic illustration of one preferred embodiment of the invention which comprises a coating chamber 2, capable of being evacuated by a vacuum pump schematically indicated at 5. A substrate 4, such as a selenium-coated aluminum plate, to be coated with cadmium is positioned on a carrier 6. A plurality of cooling coils 8 are secured to the bottom of the carrier 6 so as to insure adequate cooling of the substrate 4. The cadmium vapor source 10 comprises an inner crucible 12 for holding the molten cadmium 14. A plurality of apertures 16 are provided in the crucible 12 t'o permit escape of cadmium vapors in a downwardly direction. A plurality of heating elements 18, such as Calrod heaters, surround the inner crucible 12 so as to maintain the crucible and its contained cadmium 14 at a predetermined elevated` temperature. A layer of insulating material 20 preferably surrounds the `heating elements. The top portion 22 of the cadmium vapor source 10 is preferably removably secured thereto to permit periodic replenishing of the cadmium. A number of thermocouples (not shown) are provided so as to measure the temperature of thev heating elements 18, the temperature of the cadmium 14 and the temperature of the substrate 4. These are preferably connected to suitable automatic controllers (not shown) for regulating the power input to the heaters and coolant input to the cooling coils i-n order 'to maintain the various temperatures within the necessary predetermined tolerances.

In one preferred embodiment of the invention, a support means 26 is positioned above the cooled carrier means 6. A movable air-tight pressure chamber 28, secured to the bottom of the carrier means 6 and open to the atmosphere through passage 30, supports the carrier means 6. O rings 32 insure against leakage of air into the evacuated coating chamber 2. While the coating chamber 2 is being evacuated, the upper part 27 of the pressure chamber 28, ibearing the carrier 6 and the substrate 4, is driven like a piston by the high relative pressure existing outside the coating chamber 2 and within the pressure chamber 28, upwardly against the support 26. As a result, the substrate 4 is sandwiched ybetween the support 26 and the carrier 6, thereby insuring intimate heat transfer contact between the substrate 4 and the cooling surface of the carrier 6; subsequently, when the vacuum in the coating chamber Z'is broken, the upper part of the pressure chamber 27, along with the carrier 6 and` substrate 4, will return to its rest position as indicated by dotted lines. The coated substrate can be unloaded and` the chamber can be reloaded with an` uncoated substrate by separating the coating chamber 2,

into two sections of suitable joints indicated at 3, withdrawing the coated substrate and inserting an uncoated substrate.

In another embodiment of the invention, the support `means 26 comprises a grid which delines many small squares or any other desired shapes. In this way, it is possible to mask certain areas of the substrate during the coating operation and thereby obtain a plurality of small coated areas of a desired size deiined by Strips of uncoatedl substrate. The large substrate can then 4be cut along the uncoated strips thereby producing a plurality of small coated plates of any desired shape.

In another preferred embodiment of the invention, a small inner crucible may ybe suitably supported in intimate heat transfer relationship within the cadmium vapor source 10. 'Ihis inner crucible has substantially less mass than the cadmium vapor source 10 and is charged with only enough cadmium to complete one cycle of the coating operation. Therefore, during the coating run, the cadmium is evaporated to dryness. When subsequent to the run, the coating chamber 2 is opened to the atmosphere, the inner crucible is removed from the cadmiurn coating source 10 by means of the closure 22. A cold crucible bearing a similar charge of cadmium is then placed within the Vapor source 10. In this way, the chamber can be opened immediately following each coating run without the necessity of rst cooling the cadmium vapor source to avoid the oxidation of the cadmium charge. Also there is subsequently no necessity to reheat the cadmium vapor crucible 10 lsince it can be maintained at la constant temperature. Therefore, only a relatively short time is required to heat the relatively low-mass inner crucible to a temperature above the vaporization temperature of the cadmium it contains. As a result, time losses between the actual coating cycles are reduced to -a minimum.

While the present invention is particularly designed for the production of cadmium rectifier plates, which have an intermediate layer of selenium, it is also applicable to numerous other uses. For example, uncoated plates can lbe coated with only cadmium. In that case, cooling of the substrate would be entirely unnecessary; this would eliminateV the need for the grid and the pressure chamber since intimate contact between the substrate and the carrier would then be entirely unnecessary. Also, although only cadmium was specifically mentioned as a coating material, other materials, such as Selenium, zinc and the like could equally well be used.

Since certain changes may be made in the above apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. Apparatus for coating a substrate with a material such as cadmium and the like by vapor deposition techniques which comprises, a vacuum-tight coating chamber, means for permitting loading of said chamber, means for evacuating said chamber, means for providing a crucible containing said coating material within said chamber, said crucible being provided with walls defining a plurality of apertures which serve to direct the cadmium coating vapors downwardly toward a substrate to be coated, means for heating said crucible and thereby vaporizing said coating material, means for providing a carrier means for said substrate, means for cooling said carrier means, means for positioning a support means above said substrate, and means for moving said carrier means up to said support means in order to grip said substrate between said support means and said carrier means, thereby insuring an intimate heat transfer relationship between said substrate and said cooled carrier means, said moving means comprising a pressure chamber having amovable element, said element being movable in response to the pressure differential created between the atmosphere and the evacuated coating chamber.

2. Apparatus for coating a substrate with a vaporizable metal such as cadmium and the like by Vapor deposition techniques which comprises a vacuum-tight coating chamber, means for permitting loading of said chamber, means for evacuating said chamber, means for providing a crucible containing said vaporizable metal within said chamber, said crucible being provided with walls dening a plurality of apertures which serve to direct the coating 10 vapors downwardly toward a substrate to be coated, means for heating said Crucible and thereby vaporizing said coating material, means for providing a carrier means for said substrate, means for cooling said carrier means,

means for positioning a support means above said subl5 2,354,521

strate, and means for moving said carrier means up to said support means in order to grip said substrate between said support means and said carrier means, thereby insuring an intimate heat transfer relationship between said substrate and said cooled carrier means, said moving means comprising a piston-like pressure chamber having a movable'element, said movable element being attached to the bottom of said carrier means and being capable of movingsaid carrier means upwardly in response to the pressure differential created subsequent to the evacuation of said coating chamber.

References Cited in the le of this patent UNITED STATES PATENTS 1,994,668 Russell Mar. 19, 1935 Hewlett July 25, 1944 2,778,485 Gabbrielli Jan. 22, 1957 

1. APPARATUS FOR COATING A SUBSTRATE WITH A MATERIAL SUCH AS CADMIUM AND THE LIKE BY VAPOR DEPOSITION TECHNIQUES WHICH COMPRISES A VACUUM-TIGHT COATING CHAMBER, MEANS FOR PERMITTING LOADING OF SAID CHAMBER, MEANS FOR EVACUATING SAID CHAMBER, MEANS FOR PROVIDING A CRUCIBLE CONTAINING SAID COATING MATERIAL WITHIN SAID CHAMBER, SAID CRUCIBLE BEING PROVIDED WITH WALLS DEFINING A PLURALITY OF APERTURES WHICH SERVE TO DIRECT THE CADMIUM COATING VAPORS DOWNWARDLY TOWARD A SUBSTRATE TO BE COATED, MEANS VAPORS DOWNWARDLY TOWARD A SUBSTRATE TO BE COATED, MEANS COATING MATERIAL, MEANS FOR PROVIDING A CARRIER MEANS FOR SAID SUBSTRATE, MEANS FOR COOLING SAID CARRIER MEANS MEANS FOR POSITIONING A SUPPORT MEANS ABOVE SAID SUBSTRATE, AND MEANS FOR MOVING SAID CARRIER MEANS UP TO SAID SUPPORT MEANS IN ORDER TO GRIP SAID SUBSTRATE BETWEEN 